1. Field of the Invention
The present invention relates to a traction battery management system, and more specifically to a battery management system for an electric vehicle, provided with various functions such as a remaining battery capacity display function, charger control function and battery life estimating function, etc.
2. Description of the Prior Art
In an electric vehicle developed recently, it is important to detect various battery conditions accurately, in particular to obtain a remaining battery capacity accurately. Therefore, various techniques related thereto have been so far proposed.
For instance, Japanese Published Unexamined (Kokai) Patent Application No. 6-6901 discloses such a remaining battery capacity meter of current integrating type that a reference remaining battery capacity is obtained by subtracting a consumed watt-hour (energy) corrected according to various operating conditions from an initial battery capacity and further the obtained reference remaining battery capacity is corrected.
Further, Japanese Published Unexamined (Kokai) Patent Application No. 5-276603 discloses such a battery management technique that a remaining battery capacity meter of current integrating type is provided to obtain an accurate remaining battery capacity on the basis of charge and discharge current, and that the traction battery is controllably charged according to data indicative of detected discharged watt-hour, the number of times of charges, the motor drive data, etc. all stored in a memory.
In the conventional remaining battery capacity meter of current integrating type, however, since an error inevitably produced and further accumulated whenever the remaining battery capacity is obtained, there exists a problem in that the accumulated error must be corrected.
By the way, in the electric vehicles, since a relatively high voltage is required as a power source for driving the vehicle, the traction battery used as the primary power source is usually formed by a combination battery such that a plurality of module batteries each composed of a plurality of cells are combined with each other. In the case of a lead acid battery, for instance, a module battery of rated voltage 12 V (.apprxeq.2.1 V.times.6) is composed of six cells of 2.1 V, and a traction battery of rated voltage 336 V (=12 V.times.28) can be obtained by connecting 28 unit batteries in series. Therefore, the total number of the traction battery cells is as large as 168 (=6.times.28).
As a result, when a plurality of series-connected batteries are charged, in order to charge the combination battery uniformly (without producing any difference in charged capacity among the respective unit batteries or cells), it is necessary to charge the traction battery uniformly and periodically (at regular intervals).
In other words, the charged capacities of the respective battery units or cells are different from each other due to the difference in characteristics among the battery cells, when the traction battery is repeatedly charged in the ordinary way, there inevitably exist some battery cells not charged sufficiently. Therefore, in order to charge all the battery cells sufficiently (without producing any insufficiently charged cells or units), it is necessary to charge the traction battery at regular intervals at a constant lower current for a longer time than the current and time of the ordinary charge, which is referred to as equalizing charge for eliminating the insufficiently charged battery cells.
For instance, when the battery capacity C is 90 Ah and therefore the traction battery is charged by a current 0.1 C (=9 A) for a predetermined time under the ordinary charge condition, it is necessary to further charge the traction battery by a constant small current 0.05 C (=4.5 to 1 A) for two hours periodically, continuously after the ordinary battery charge.
On the other hand, when the traction battery is additionally charged by a constant small current, since the battery cells are overcharged partially, when the equalizing charge is effected excessively, there arises a problem in that the battery life is shortened.
Conventionally, therefore, the traction battery has been so far equalizing charged manually, in dependence upon the number of times of charges or the used time period, thus causing the maintenance of the electric vehicle to be complicated.
In addition, since the life of the traction battery mounted on an electric vehicle is also presumed by the worker on the basis of the number of times of chargings or the used time period, there exists a need of estimating the battery life easily or automatically on the basis of the various battery data.